Thanks for the clear explanation 2018-04-05 11:08 GMT+02:00 T.J. Crowder <[email protected]>:
> On Thu, Apr 5, 2018 at 9:29 AM, Cyril > Auburtin <[email protected]> wrote: > > > > I don't understand: > > > > ```js > > > > var z=1; var fn = (z = z) => z; fn() > > // throws ReferenceError: z is not defined > > ``` > > SpiderMonkey's error message is a bit clearer here: > > > ReferenceError: can't access lexical declaration `z' before > initialization > > Remember that default parameter values have their own scope. In that > scope, they have a TDZ (like `let` and `const` do in a block) until they're > initialized. I believe this is covered in > [FunctionDeclarationInstantiation][1] > Step 28. At the point the default value expression `= z` is evaluated, the > `z` parameter has been created as an uniniitialized binding (by > [CreateMutableBinding][2] in 28.f.i.2). Consequently, it tries to get the > value of that uninitialized binding, which results in a ReferenceError as > described in Table 14 under [Environment Records][3]: > > > > GetBindingValue(*N*, *S*) - Returns the value of an already existing > binding from an Environment Record. The String value *N* is the text of the > bound name. *S* is used to identify references originating in strict mode > code or that otherwise require strict mode reference semantics. If *S* is > true and the binding does not exist throw a ReferenceError exception. **If > the binding exists but is uninitialized a ReferenceError is thrown**, > regardless of the value of *S*. > > *(My emphasis.)* > > E.g., you get that error for the reason you get that same error from: > > ```js > var z = 42; { console.log(z); let z; } > ``` > > -- T.J. Crowder > > [1]: https://tc39.github.io/ecma262/#sec-functiondeclarationinstantiation > [2]: https://tc39.github.io/ecma262/#sec-declarative-environment-records- > createmutablebinding-n-d > [3]: https://tc39.github.io/ecma262/#sec-environment-records > >
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